Filter JPEG files in MACOSX

[paolo]

Filter jpeg files in MACOSX

How can I filter the files created by the application Preview  and  
saved as JPEG?
According to the Rev documentation I should use either there two  
commands:

    answer file "SELEZIONA" of type "JPEG"

    answer file "SELEZIONA" with filter "JPEGs,*.jpg"

Unfortunately both of them do not work .As far as I know, the first  
one does not work because  the application  Preview from Apple does  
not set the filetype to JPEG even if you choose to save the image as  
JPEG.  (crazy)

The second one does not work because  even if the file has the  
extension .jpg,  in the contextual menu the extension of that file  
disappear.
Why?

To me, this file type thing in MacOSX is a mess.